1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machines, printers and facsimiles, in which image formation is carried out in an electrophotographic process.
2. Description of the Related Art
Some image forming apparatuses based on the electrophotographic process use a two-component magnetic brush phenomenon method, the development method for making an electrostatic latent image on a photosensitive body visible, in which a two-component developer including an insulating toner and magnetic carriers is mixed and agitated and in which the magnetic carriers electrostatically attracting the insulating toner are magnetically attracted in the form of brush to a circumferential surface of a development roller by magnetic forces from magnetic poles in the development roller so that the developer carried on the development roller is transferred onto a surface of the photosensitive body as the development roller rotates. This method is widely employed particularly in a color image forming system that produces one color image through a plurality of electrophotographic processes using different color toners.
In the image forming based on the electrophotographic process using the two-component magnetic brush phenomenon method, however, when there are two continuous image areas in an image that have different densities, a phenomenon may occur in which an image density of one image area at the boundary with the other image area decreases.
For example, as shown in FIG. 13A, when an image changes from a half-tone area G1 to a background area G2 in a sub-scan direction Y (opposite the paper feed direction) perpendicular to the main scan direction X of an exposure beam for forming an electrostatic latent image on the surface of the photosensitive body, a sub-scan direction rear end part G1a of the half-tone area G1 which adjoins the half-tone area G2 may decrease in density. Further, as shown in FIG. 13B, when an image changes from a low-density area G3 to a high-density area G4 in a sub-scan direction Y, a sub-scan direction rear end part G3a of the low-density area G3 adjoining the high-density area G4 may decrease in density.
First, the density reduction in the rear end part of the half-tone area adjoining the background area is explained with reference to FIGS. 14A and 14B. FIG. 14A shows a front edge part of a latent image of the half-tone area formed on the photosensitive body in contact with a developer layer. FIG. 14B shows a rear end part of the latent image of the half-tone area in contact with the developer layer. To a development roller 102 is applied a development bias (e.g., xe2x88x92500V). The surface of a photosensitive drum 101 is charged by a charger 103 to a bias (e.g., xe2x88x92650V) higher in absolute value than the development bias. The potential of the latent image S1 of the half-tone area is changed to a potential (e.g., xe2x88x92200V) lower in absolute value than the development bias by an exposure beam L.
As shown in FIG. 14A, when the front edge part S1a of the latent image S1 contacts the developer layer 104 formed over the circumferential surface of the development roller 102, a forward development electric field acts on the toner tq present at a contact position Q between the surface of the photosensitive drum 101 and the developer layer 104, which toner tq is attracted to the surface of the developer layer and then to the surface of the photosensitive drum 101. When as shown in FIG. 14B the rear end part of the latent image S1 contacts the developer layer 104, a latent image S2 of the background area comes near the developer layer 104, with the result that a reverse development electric field which repels the toner tb away from the surface of the developer layer 104 down toward the circumferential surface of the development roller 102 acts on the toner tb present at a position in the developer layer 104 facing the rear edge part S1b of the latent image S1.
The toner tb submerged toward the circumferential surface of the development roller 102 moves toward the surface of the developer layer 104 as the contact position Q approaches as a result of rotation of the development roller 102, but there is a time delay before it reaches the surface of the developer layer 104. Hence, the rear end part of the latent image S1 of the half-ton area that adjoins the latent image S2 of the background area is not attached with a sufficient amount of toner, resulting in a reduced image density at the rear end part of the half-tone area in the image.
In a case where there is a latent image S2 of the background area in front of the latent image S1 of the half-tone area as shown in FIG. 14A, when the front edge part S1a of the latent image S1 of the half-tone area is situated at the contact position Q, the toner tf that is repelled from the surface of the developer layer 104 by the latent image S2 of the background area at the front exists in the developer layer 104. However, as the development roller 102 rotates, the toner tf moves away from the contact position Q and the toner tq that is attracted to the surface of the developer layer 104 by the low potential of the latent image S1 of the half-tone area immediately comes close to the contact position Q and adheres to the latent image S1. Therefore, the front end part of the half-tone area adjoining the background area in the image does not produce a reduction in the image density.
Next, the density reduction in the rear end part of a low-density area adjoining a high-density area will be explained by referring to FIGS. 15A-15C. FIG. 15A shows a front edge part of a latent image of the low-density area formed over the photosensitive body in contact with the developer layer. FIG. 15B shows a rear end part of the latent image of the low-density area in contact with the developer layer. FIG. 15C shows a latent image of a high-density area situated behind the latent image of the low-density area in contact with the developer layer. To the development roller 102 is applied a development bias (e.g., xe2x88x92500V). The surface of the photosensitive drum 101 is charged by the charger 103 to a potential (e.g., xe2x88x92650V) higher in absolute value than the development bias. The potential of a latent image S3 of the low-density area is made lower in absolute value (e.g., xe2x88x92300V) than the development bias by an exposure beam L. The potential of a latent image S4 of the high-density area is made lower in absolute value (e.g., xe2x88x92200V) than that of the latent image S3 of the low-density area by the exposure beam L.
With the front edge part S3a of the latent image S3 of the low-density area in contact with the developer layer 104 of the development roller 102 as shown in FIG. 15A, the toner ta present at the contact position Q between the surface of the photosensitive drum 101 and a forward development electric field acts on the circumferential surface of the developer layer 104, which attracts the toner ta toward the surface of the development roller 102, allowing it to adhere to the surface of the development drum 101. Thus, the toner tc adheres to the entire surface of the latent image S3 of the low-density area formed over the surface of the photosensitive drum 101 as shown in FIG. 15B, until a rear edge part S3b of the latent image S3 of the low-density area reaches the contact position Q.
After this, when the latent image S4 of the high-density area located behind the latent image S3 of the low-density area comes to the contact position Q and begins to contact the developer layer 104, as shown in FIG. 15C, a stronger development electric field is produced in the forward direction between the latent image S4 and the developer layer 104 than that between the latent image S3 and the developer layer 104 because the potential of the latent image S4 is lower in absolute value than that of the latent image S3. This causes a larger amount of toner te to adhere to the latent image S4 than that of the latent image S3. Hence, at and around the position in the developer layer 104 facing the contact position Q, the carriers are deprived of most of the toner covering their surfaces, which are then exposed, with the result that the charged potential of the carriers attracts the toner tc from the rear end part of the latent image S3, to which once it has adhered, back to the developer layer 104. As a result, the rear end part of the latent image S3 of the low-density area adjoining the latent image S4 of the high-density area is not attached with a sufficient amount of toner, reducing the image density of the rear end part of the low-density area in the image.
As described above, the density reduction in the rear end part of the low-density area adjoining the high-density area is caused by a large amount of toner attaching to the latent image S4 of the high-density area immediately following the latent image S3 of the low-density area, followed by the toner, which has once attached to the latent image S3 of the low-density area, being drawn back to the developer layer 104 by the potential of the carriers in the developer layer 104 that have lost the toner. Hence, where a high-density area lies immediately before the low-density area, the front end part of the low-density area adjoining the high-density area does not undergo a reduction in the image density.
Such image density reductions in the rear end part of a half-tone area and in the rear end part of a low-density area are rather conspicuous in figure images formed by image generation apparatus such as personal computers which are increasing in numbers in recent years. In the image forming apparatus based on an electrophotographic system, particularly in printers connected to the image forming apparatus via network, there are stronger demands than in copying machines for preventing such image density reductions in the rear end part of a half-tone area and in the rear end part of a low-density area.
Thus, conventional image forming apparatus have provisions which, as disclosed in Japanese Unexamined Patent Publications JP-A 5-281790 (1993) and JP-A 6-87234 (1994) for example, increase the precision of a laser scan unit, which forms an electrostatic latent image on the surface of the photosensitive body, and adjust parameters of a development unit, which makes the electrostatic latent image visible, to enhance the contrast of the development electric field, thereby preventing the image density reductions in the rear end part of a half-tone area and in the rear end part of a low-density area.
The method of enhancing the contrast of the development electric field by increasing the precision of the laser scan unit, however, has a drawback of increasing the size and cost of the image forming apparatus. Further, when the number of scan lines in the sub-scan direction is to be increased for enhancement of the image resolution, the reduced contrast of the development electric field makes more conspicuous the image density reductions in the rear end part of the half-tone area adjoining the background area and in the rear end part of the low-density area adjoining the high-density area. It is therefore difficult to achieve both the image resolution enhancement and the prevention of partial image density reductions.
Further, because the image forming process based on the electrophotographic system has a variety of parameters of a plurality of units acting on one another in a complicated manner, it is very difficult to analyze the physical properties of the units and determine the parameters for preventing the image density reductions. Directly measuring the physical properties of the units by using measuring devices is not easy. Further, there are characteristic variations among different image forming apparatus due to individual differences. Characteristic variations of the units, which will cause image density reductions, are also produced by external environmental changes such as temperature and humidity and by progressive degradation over time of parts making up the apparatus. Considering these, it is all the more difficult to determine a unique set of characteristics capable of preventing the image density reductions.
In the arrangement disclosed in Japanese Unexamined Patent Publication JP-A 10-65920 (1998), therefore, the process for preventing image density reduction involves outputting measurement data consisting of an array of toner patches with different numbers of pixels to be corrected (ranges of a rear end part where a density reduction takes place) and different pixel-value correction amounts (correction amounts corresponding to the density reductions), determining from the output results an appropriate number of pixels to be corrected and an appropriate amount of pixel-value correction, storing them in a characteristic description means, extracting from input image data a rear edge area where a loss of image (a partial reduction in the image density) may occur, and correcting the image data in the extracted rear edge portion based on the number of pixels to be corrected and the amount of pixel-value correction, both held in the characteristic description means, thereby preventing image density reduction in that area.
In the arrangement disclosed in JP-A 10-65920, however, because the toner patches with a plurality of density levels (2-256 levels) are formed by the image output apparatus and the degree of the decrease in density in the rear end part of the image is calculated for each density level, the processing takes much time and a large amount of toner is required to form a plurality of toner patches.
An object of the invention is to provide an image forming apparatus which can easily determine characteristics which cause image density reductions without having to analyze physical properties of constitutional units making up the image forming apparatus and which can prevent image density reduction in a rear end part of a half-tone area adjoining a background area and in a rear end part of a low-density area adjoining a high-density area and thereby form an image with an appropriate density at all times irrespective of individual differences among different apparatus, external environmental changes and gradual deterioration over time of the apparatus.
In order to solve the problems described above, the invention provides an image forming apparatus in which image formation is carried out by an electrophotographic process based on a predetermined image forming condition, the image forming apparatus comprising:
an optical sensor for detecting densities of a toner patch image formed on a surface of a photosensitive body and outputting electric signals corresponding to the detected image densities; and
a control unit for changing a set value of an image forming condition according to a degree of deflection in output signals of the optical sensor corresponding to detected densities of a rear edge part of the toner patch image.
In this configuration, the set value of the image forming condition is changed according to the degree of deflection in the optical sensor output signals representing the rear edge part of the toner patch image formed on the surface of the photosensitive body during the process control. When a decrease in image density occurs at the rear edge part of the toner patch image, a deflection is produced in the output signals of the optical sensor according to the degree of the decrease in image density. Hence, the degree of the decrease in image density that occurs at the rear edge part of the image is measured by the degree of deflection in output signals of the optical sensor. The set value of the image forming condition is changed so as not to produce a decrease in image density at the rear edge part of an image. The process control is ordinary processing performed by the image forming apparatus to set the image forming conditions in an image forming apparatus. Thus, without having to add new components to an image forming apparatus, it is possible to prevent decrease in image density at the rear edge part of an image.
In the invention it is preferable that the control unit compares the degree of deflection in the output signals of the optical sensor for the rear edge part of the toner patch image with a low-output side reference value of deflection and a high-output side reference value of deflection and changes the set value of the image forming condition only in a range between values of the image forming condition corresponding to the low-output side reference value of deflection and the high-output side reference value of deflection.
In this configuration, the set value of the image forming condition is changed according to the degree of deflection in the optical sensor output signals, in the range between values corresponding to the low-output side reference value of deflection and the high-output side reference value of deflection. Hence, the image forming condition is not changed excessively beyond the predetermined allowable range. This prevents decrease in image density at the rear edge part of an image without incurring an increase in cost and size of the apparatus due to increased capacities of constitutional devices which would result from an excess image forming condition, or without causing significant degradation of image quality.
In the invention it is preferable that the control unit increases or decreases a difference between a charged potential and a development potential on the surface of the photosensitive body according to the degree of deflection in the output signals of the optical sensor.
In this configuration, the difference between the charged potential and the development potential on the surface of the photosensitive body is set to decrease as the degree of deflection in output signals of the optical sensor that corresponds to the degree of the decrease in image density at the rear edge part of the toner patch image increases. Hence, by changing the difference between the charged potential and the development potential on the surface of the photosensitive body, the decrease in density at the rear edge part of an image can be prevented.
The control unit can change the difference between the charged potential and the development potential on the surface of the photosensitive body by controlling the grid voltage applied to the charger.
With this configuration, the difference between the charged potential and the development potential on the surface of the photosensitive body can be changed relatively easily and precisely to prevent the decrease in density at the rear edge part of an image by controlling the operation of the power supply device which supplies the grid voltage to the charger.
In the invention it is preferable that the control unit increases or decreases an quantity of exposure light applied to the surface of the photosensitive body according to the degree of deflection in the optical sensor output signals.
In this configuration, the quantity of the exposure light applied to the surface of the photosensitive body is set to increase as the degree of deflection in the output signals of the optical sensor that corresponds to the degree of the decrease in image density at the rear edge part of the toner patch image increases. Hence, changing the quantity of the exposure light applied to the photosensitive body surface can prevent decrease in density at the rear edge part of an image.
The control unit can change the quantity of the exposure light applied to the surface of the photosensitive body by controlling at least one of a drive power applied to the exposure light source, a PWM value of a drive pulse applied to the exposure light source, an exposure speed and an exposure light spot diameter.
With this configuration, the quantity of the exposure light applied to the surface of the photosensitive body can be changed relatively easily and precisely to prevent decrease in density at the rear edge part of an image by controlling the operation of the drive circuit that drives the exposure light source.
In the invention it is preferable that the control unit increases or decreases a quantity of discharge light applied to the surface of the photosensitive body according to the degree of deflection in the output signals of the optical sensor.
In this configuration, the quantity of the discharge light applied to the surface of the photosensitive body is set to increase as the degree of deflection in the output signals of the optical sensor that corresponds to the degree of the decrease in image density at the rear edge part of the toner patch image increases. Hence, the decrease in density at the rear edge part of an image can be prevented by changing the quantity of the discharge light applied to the surface of the photosensitive body.
The control unit can change the quantity of the discharge light applied to the surface of the photosensitive body by controlling the voltage applied to the discharge light source.
With this configuration, the quantity of the discharge light applied to the surface of the photosensitive body can be changed relatively easily and precisely to prevent the decrease in density at the rear edge part of an image by controlling the operation of the drive circuit that drives the discharge light source.
In the invention it is preferable that the control unit increases or decreases a speed of image development on the surface of the photosensitive body according to the degree of deflection in the optical sensor output signals.
In this configuration, the image development speed on the surface of the photosensitive body is set to decrease as the degree of deflection in the optical sensor output signals that corresponds to the degree of the decrease in image density at the rear edge part of the toner patch image increases. Hence, the density reduction at the rear edge part of an image can be prevented by changing the image development speed on the surface of the photosensitive body.
The control unit can change the quantity of the exposure light applied to the surface of the photosensitive body by controlling the rotation speed of the development roller.
In this configuration, the image development speed on the surface of the photosensitive body can be changed relatively easily and precisely to prevent the density reduction at the rear edge part of an image by controlling the operation of the drive circuit that drives the development roller.